This invention relates to an electrochromic device which utilizes a combination of two types of electrochromic materials one of which takes on color in its electrochemically oxidized state while the other takes on color in its electrochemically reduced state, and more particularly the invention is an improvement in the electrolyte solution in the electrochromic device.
Conventional electrochromic (EC) materials are classified into two types, which will be referred to as "oxidation type" and "reduction type", respectively, in the present specification. An EC material of oxydation type takes on color in its electrochemically oxidized state, whereas an EC material of reduction type takes on color in its electrochemically reduced state.
It is known to use a combination of an oxidation type EC material and a reduction type EC material in an EC device having two oppositely arranged electrodes. The two types of EC materials are assigned to the two electrodes, respectively. For example, JP-A 59-159134 shows an EC display device using a combination of Prussian blue which assumes blue color in its electrochemically oxidized state and becomes colorless by reduction and tungsten trioxide which is colorless in its electrochemically oxidized state and assumes blue color in a reduced state. In the EC device the space between the two opposite EC electrode layers is filled with an electrolyte liquid, which is usually a solution of lithium perchlorate in propylene carbonate. U.S. Pat. No. 4,773,741 shows an EC display device of fundamentally the same type. In operation of the EC display device, electrochemical oxidation of the EC material on one electrode is accompanied by electrochemical reduction of the EC material on the other electrode. Accordingly simultaneous coloration of the two electrodes and simultaneous bleaching of the two electrodes take place. The primary purpose of this construction is intensifying blue coloration of the EC device.
In practical use of an EC device of the above type it is not seldom that the EC device is left in the bleached state (i.e. the Prussian blue electrode in the reduced state) for several days or still longer. When the EC device left in such a state is driven for coloration, the density of coloration is lower than that in the coloration operation before the last bleaching and in some cases insufficient for practical purposes. The density of coloration can be resumed to some extent by repeating coloration and bleahing many times, but for fully resuming the density of coloration it is necessary to leave the EC device in the colored state for several days. This is detrimental to the commercial value of the EC device.